It happens to me every year in late August and on through September. I start getting busy again in October, and stay busy though January, and sometimes February.
I imagine there are some cleaners who post here who have slow times that differ from those that you have. There is probably no general rule. In my opinion, it really doesn't matter why I am slow. There is nothing I can do about it as long as I am doing consistent marketing and contacting my past customers. (If you aren't, then get started.)
My advice is to just do what you know you are supposed to be doing and when the slow times come just hang on ... they always end.
Ignore the temptation to rush out and do some kind of spur of the moment marketing when these times hit you because advertising started during slow times usually fails to bring in any business. This doesn't mean you shouldn't be marketing during slow times. It just means that you shouldn't START marketing during slow times ... unless you plan on keeping it up ALL THE TIME.
If there is one single mistake that I made during my first ... say ... 15 years of doing business ... it would be not doing consistent advertising. My belief is that even bad advertising, over a long period of time, will generate some brand awareness within your market. I still have people calling my home, asking for me by name, because they can't remember the name of my business. That pretty much tells me that the quality of my WORK is reaping benefits, but the quality of my ADVERTISING is off, and needs improvement.